This little 'add-on' is in fact more a little template modification than an 'add-on'. This one removes the check for the permission that wraps the display of the 'Create Template Modification' button. The permission itself is not touched.
Somewhen between XF 1.2 RC1 and 1.2 Final they went out and disabled that button for everyone unless they are in debug mode. So if you do not want to fiddle around with your address bar, you can install that addon.